(1) Field of the Invention
The present invention relates to a fusing device and an image forming apparatus using the fusing device, and in particular relates to a fusing device that is made of a pair of roller elements functioning as a fusing roller and a pressing roller, to pass a recording medium with an unfixed toner image formed thereon to the contact nip where these roller elements are put in contact with each other, to thereby fix the toner image to the recording medium, as well as relating to an image forming apparatus using this fusing device.
(2) Description of the Prior Art
Conventionally, in image forming apparatus using electrophotography, such as facsimile machines, printers, etc., an image is output on a recording medium by charging a rotationally driven photoreceptor drum by a charger, forming an electrostatic latent image on the photoreceptor drum by irradiation with light in accordance with image information, applying toner to this electrostatic latent image from a developing unit to form a toner image, then transferring the toner image to a sheet material, paper or other recording media.
This image forming apparatus includes a fusing device that fuses and fixes the unfixed toner on the recording medium being conveyed. This fusing device, in most cases, is a thermal fusing roller type which is comprised of a pair of roller members, i.e., a fusing roller (heat roller) including a fusing source (heat generator) such as a halogen lamp and a pressing roller, arranged opposing each other at the position where the unfixed toner is fused and fixed. In this arrangement, the recording medium with an unfixed toner image thereon is passed through the press-contact area (fusing nip portion) where the two roller members press each other at a predetermined pressure, whereby the toner image on the recording medium is fixed to the recording medium (see patent document 1: Japanese Patent Application Laid-open 2007-248654).
However, the above conventional fusing device suffers the problem that pressing the roller members against each other for many hours by use of a pressing member such as a spring etc., causes partial permanent deformation in the elastic layer formed on the outer peripheral surface of the roller member.
As the measures against this problem, a prior art technique has been disclosed in which a fusing device includes a fusing roller having a thermal conductive member (metal core) that transfers heat from a heat generator to recording medium by thermal conduction and a pressing roller that has an elastic layer forming a nip portion with the fusing roller to convey the recording medium by nipping it, and is constructed such that while the distance from the thermal conductive member to the axis of the pressing roller is made constant regardless of whether the fusing device is activated or deactivated, the nip width during the fusing device being in operation is created by thermal expansion of the pressing rollers to thereby prevent partial deformation of the elastic layer of the pressing roller, which would arise when a spring or any other pressing element is used to impart a pressing force (see patent document 2: Japanese Patent Application Laid-open H08-22214).
However, in this prior arts the nip portion is created by causing the roller member to thermally expand without use of any pressing element such as a spring or the like, it was hence difficult to keep the pressing force and the nip width constant at any time. As a result there has been the problem that the fusing operation is unstable.